In controlling an AC synchronous motor with a servocontrol, the so-called vector control is generally used, in which electric current is resolved into a d-axis component—a current in the direction of a motor flux, and a q-axis component—a current in a torque direction orthogonal to the d-axis, whereby each of the current components is controlled. In order to perform the vector control, it is necessary to precisely sense a position of the magnetic pole.
Linear motors are often used combined with incremental encoders; however, an incremental encoder only senses relative positions, so that it is necessary to sense an initial position of the magnetic pole. When an initial magnetic pole position is sensed inaccurately, an axis error phenomenon occurs in which a true magnetic pole position of a motor does not coincide with a magnetic pole position sensed by a control system.
On the other hand, rotating motors are generally used combined with absolute encoders, so that their magnetic pole position is sensed and stored in advance. When an absolute encoder is used, it is not required to sense the initial magnetic pole position; however, installation errors or the like will cause the axis error phenomenon. The axis error phenomenon results in deterioration of accuracy in controlling torques, reduction of the maximum torque to be generated and the like.
Background art documents are listed below.    Non-Patent Document 1: the institute of electrical engineers of Japan, transactions on industry applications, vol. 122, No. 9, 2002    Patent Document 1: Japanese Patent Laid-Open No. 2002-247881
Non-Patent Document 1 discloses a technique, “Magnetic Pole Position Detection Method and Control of Brushless DC Servomotor with Incremental Encoder” in the institute of electrical engineers of Japan, transactions on industry applications, vol. 122, No. 9, 2002. The technique uses such a principle that when a d-axis position of a true magnetic pole position of a controlled motor (hereinafter, referred to as a true magnetic pole position of a motor), coincides with that of a magnetic pole position targeted by a controlling system (hereinafter, referred to as a target magnetic pole position), a d-axis electric current being applied does not generate torque and that when the true magnetic pole position does not coincide with the target magnetic pole position, the d-axis electric current being applied generates torque in accordance with the quantity of the axis error. More specifically, a signal for magnetic pole position detection is applied as a current command; then, a torque generated by the difference between the target magnetic pole position and the true magnetic pole position of the motor moves a motor mover, the speed of which is sensed; and then, the sensed speed is proportionally and integrally compensated to be used for correcting a phase for coordinate transformation; whereby, the quantity of the axis error can be finally converged to zero, so that the magnetic pole position can be estimated.
However, when there exist disturbances, or especially when there exists large static friction, a torque for estimating magnetic pole position is hidden in the friction torque; therefore, a problem is that a large error remains in its estimation. To solve this problem, the technique proposes that the amplitude of the signal for the magnetic pole position detection is made larger so that the estimation error is reduced; however, the proposed technique leads to other problems such as larger movements of the motor mover and louder noises during the estimation. Furthermore, amplifier capacity limits the amplitude of the current; there is accordingly a limit in the technique that the amplitude of the signal for the magnetic pole position detection is made larger in order to reduce its estimation error, which may not solve the problem.
On the other hand, there is another technique disclosed in Japanese Patent Laid-Open No. 2002-247881, as follows. In this technique, a current for detecting a magnetic pole position is applied to the motor; the position information of the magnetic pole attracted toward the magnetic flux generated by the current is obtained to estimate the magnetic pole position on the basis of the information. In this case, the phase of the applied current is shifted from a reference phase, to the reference phase +180 degrees, to the reference phase −90 degrees, and to the reference phase +90 degrees, and every magnetic pole movement caused thereby is sensed to estimate the magnetic pole position. Calculations of the magnetic pole position are performed using sensed values each obtained after the phase of the current for detection is shifted to the reference phase −90 degrees and to the reference phase +90 degrees, which can reduce accuracy degradation caused by static friction in sensing the magnetic pole position.
However, there remain problems below. Firstly, the technique uses a procedure in which a current with a phase is applied only during an appropriately short time and immediately after that the phase is shifted within a short time to move the magnetic pole move less, and does not use a method in which the quantity of motor mover movement is controlled by a feedback control loop with respect to its position or speed; therefore, a problem is that the actual quantity of motor mover movement cannot be sufficiently reduced in such a condition that friction is extremely small in comparison with the amplitude of an estimation-use signal. Furthermore, because this technique does not include convergence operations such as a feedback control, it cannot use such a procedure as switching to the next operation set after convergence of movement; thus, it becomes necessary to set a time appropriate for switching, but a problem is that it is difficult to adjust that time.
Secondly, the document of the technique explains that by sequentially issuing a pair of commands for diagonal phases of the magnetic field, a rotation amount becomes very small and a rotational position returns back; however, because the operations sensing a magnetic pole position do not include a feedback control loop with respect to the magnetic pole position or speed, the rotation position does not return back to the original position by a single set of operations for detection; then, a problem is that the magnetic pole position after the detection is not guaranteed to be the same as that before the detection. When there exist different disturbances, for example, in a positive direction and a negative one, there arises a problem that motor mover positions become different before and after its detection. Especially when the detection operation set is repeated a number of times, there arises a problem that the magnetic pole position is shifted gradually. The document of the technique describes that a position/speed control unit is incorporated in the configuration so as to solve the problem; however, in an ordinary position/speed control loop, when the magnetic pole position is displaced, the control loop does not normally operate, so that the configuration described above cannot solve the problem.
Thirdly, because the technique uses an arctangent function to estimate the magnetic pole position, a problem is that its calculation load is heavy so that a lot of calculation time is required.